1. Field of the Invention
This present invention relates to a porous silica perform and method for fabricating a porous silica preform.
Priority is claimed on Japanese Patent Application No. 2003-381074 filed Nov. 11, 2003, and Japanese Patent Application No. 2003-391025, filed Nov. 20, 2003, the contents of which are incorporated herein by reference.
2. Description of Related Art
A method for fabricating a fluorine-doped porous silica preform has been proposed in which glass forming gas and fluorine-containing compound gas are supplied to a glass synthesizing burner, silica soot, e.g., fine particles of glass, ate synthesized by hydrolysis or oxidation in an oxyhydrogen flame, and the resultant glass particles are deposited on a staring rod to form a porous silica preform (for example, Vapor-phase Axial Deposition (VAD) method, see Japanese Unexamined Patent Application, First Publication Nos. S59-232934 and H7-330366).
In this method for fabricating a porous silica preform, it is well-known that if the fluorine-containing compound gas is present at a high concentration, the following etching reaction occurs in glass particles and SiF4 is generated as an adsorbed species:                SiO2 (s)+4F (g)SiF4 (ad)+O2 (g), where “s”, “g”, and “ad” stand for solid, gas, and adsorbed species, respectively.        
The resultant SiF4 weakens the adhesion force between glass particles. Thus, if the fluorine-containing compound gas is present at a high concentration, adhesion force between the starting rod and the glass particles is weakened when glass particles are deposited directly on a starting rod. As a result, the porous silica preform frequently falls off from the starting rod, resulting in a poor yield.
Furthermore, since SiF4 weakens the adhesion force between glass particles, breakage (soot breakage) may frequently occur in a porous silica preform, resulting in a poor yield.
Such a soot breakage tends to occur more frequently in a tail (tip) of a porous silica preform regardless of absence or presence of fluorine dopant when the porous silica preform is cooled after production.
Accordingly, another method for fabricating a porous silica preform was proposed in which upon completion of fabricating a porous silica preform, the tempera of the flame of the burner is increased so that a hardening layer is formed on the surface of an end of a porous silica preform (see Japanese Patent No. 2999095).
Although JP 2999095 teaches that soot breakage can be reduced by heating the surface of a porous silica preform so that the powder density of the hardened layer is no less than 0.3 g/cm3 and no more than 0.5 g/cm3, it does not address issues of a fluorine-doped porous silica preform.
When fabricating a fluorine-doped porous silica preform, an incidence of soot breakage may not be suppressed since a hardened layer may not be effective since the adhesion force between glass particles is reduced, as described above.
As will be discussed later, soot breakage tends to occur when the temperature of deposited glass particles decreases and the glass particles undergo thermal contraction:
(1) Typically, glass particles are synthesized and deposited on a starting rod using plural glass synthesizing burners. Since a boundary between deposition areas of the adjacent burners is distant from the center of the oxyhydrogen flame from a burner, the temperature of the boundary heated by the oxyhydrogen flame is lower. When glass particles which have been deposited on the starting rod pass through such a boundary between deposition areas of each of the burners, the glass particles undergo thermal contraction, resulting in soot breakage.
(2) Upon the completion of deposition of glass particles, as deposited glass particles exits from all of the deposition areas (areas heated by oxyhydrogen flame) of the burners, the temperature of the deposited glass particles decreases and the glass particles undergo thermal contraction, resulting in soot breakage.
Furthermore, as described above, a porous silica preform that is doped with fluorine at a high concentration easily cracks since the adhesion force between glass particles is weak. Therefore, when an optical fiber preform is fabricated from a porous silica preform, a porous silica preform easily cracks, especially while handling of the preform.